1. Field of the invention
This invention relates to a method for producing ferrites and more particularly, to an improvement in the method for producing composite oxides used as starting materials for making high-quality ferrites, wherein chlorides or oxides of metals constituting soft ferrites are provided as a starting material.
2. Description of the Prior Art
In general, the industrial method of production of ferrites comprises mixing compounds such as oxides or carbonates of metals such as iron, manganese, magnesium, nickel, copper, zinc and the like used to constitute ferrites, at predetermined molar ratios, subjecting the mixture to calcination (preliminary sintering), pulverizing, granulating and molding, and finally sintering at high temperatures. However, this method has the problem that it requires the use of a calcination step wherein relatively high temperature and long time are used, influencing the quality of the product such as the incorporation of impurities during the production process and non-uniformity of quality as viewed microscopically and needing a long production time and a great amount of energy.
In general, starting iron oxides used as a major proportion of compositions for making ferrite are, in most cases, obtained by subjecting hydrochloric acid pickling waste liquors generated in the hot-rolling process of steel sheets or coils to high temperature oxidation roasting using a spray roasting furnace.
After the mixing, these metal oxides are thermally heated at the time of calcination or sintering and react to finally form a single phase having a spinel structure after going through an intermediate formation phase such as ZnFe.sub.2 O.sub.4, ZnMn.sub.2 O.sub.4, MnFe.sub.2 O.sub.4 or the like.
In the above-stated series of ferrite production steps, the uniform dispersion of the respective oxides by mixing greatly influences the magnetic characteristics of a final ferrite product. Accordingly, improvement has been demanded while taking economy into consideration.
With respect to the phase change at the time of sintering, the intermediate formation phases such as ZnFe.sub.2 O.sub.4 and MnFe.sub.2 O.sub.4 are volumetrically expanded at 1.5 to 2% and 0.3 to 0.5%, respectively, during their formation. Accordingly, if the calcination step is omitted, press molding is given a cyclic stress owing to the expansion and subsequent shrinkage during sintering. As a result, the sintered products finally decrease in sintering density, involve defects such as deformation and cracks, and result in deterioration in accuracy of dimension and strength.
As an improved method for producing ferrites over the prior techniques, there has been proposed a production method wherein a mixed solution of chlorides of metals constituting ferrite is used as a starting material and roasted for oxidation (Japanese Patent Publication No. 47-11550). However, when all the starting materials are used in the form of metal chlorides, metal chlorides, such as zinc chloride having a high vapor pressure are vaporized during roasting, thus presenting the problem that such a metal can by no means reach the intended concentration in the product obtained from the bottom of the roasting furnace.
To avoid this, in Japanese Patent Publication No. 63-17776, there is proposed a method wherein, among metals constituting a ferrite, only metals whose chlorides are low in vapor pressure are provided as a mixed solution of the chlorides. This mixed solution is roasted for oxidation to obtain mixed oxides and/or partially reacted composite oxide. The oxides thus obtained are then mechanically mixed with oxides of metals whose chlorides have high vapor pressures. The mixture is directly sintered, without calcination, to obtain a ferrite. In this technique, with respect to the Zn ingredient, its oxide having a size of from 0.1 to 1 .mu.m has to be mechanically mixed in a subsequent step. This leads to non-uniformity of the composition of the mixture, causing the magnetic characteristics of a final product to be degraded. This method is pronouncedly improved over the method using the respective powdered oxides as starting materials but the improvement is not satisfactory with respect to uniform dispersion.
As stated above, in the known methods for producing composite oxides used as starting materials for soft ferrites, metal chlorides, such as of Zn, having a high vapor pressure cannot be roasted in a high temperature roasting step simultaneously with other metal chlorides. After the roasting, this involves an additional mechanical mixing step, thus leaving room for a further improvement with respect to uniform dispersion or the incorporation of impurities.